From a transmission perspective, wireless communication systems have evolved from relatively simple Single-Input Single-Output (SISO) systems, wherein a mobile device communicates with wireless access equipment over a single transmission channel, to Multiple-Input Multiple-Output (MIMO) systems supporting high transmission rate multi-media communications sharing available system resources such as transmission channels, system power, transmit and receive equipment, etcetera. In a MIMO system, both the transmit and receive sides are equipped with multiple antennas for data transmission.
MIMO systems operate in various wireless access methods, such as but not limited to Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), and the like.
With the MIMO technique, given multiple antennas, the spatial and/or the polarization dimension can be exploited to significantly improve the performance of the wireless transmission and to produce significant capacity gains over SISO systems using the same bandwidth and transmit power.
MIMO systems transmit signals from different transmit antennas and the receiving antennas receive a superposition of all the transmitted signals. Since the receiver detects the same signal several times at different positions in space at least one position should not be in a fading dip. Three main categories of MIMO can be distinguished, spatial or polarization multiplexing, diversity coding, and precoding.
In spatial or polarization multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel.
In diversity coding a single stream (unlike multiple streams in spatial or polarization multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas using certain principles of full or near orthogonal coding. Diversity exploits the independent fading in the multiple antenna links to enhance signal diversity.
Precoding, sometimes viewed as beamforming, is a scheme wherein the same signal is emitted from each of the transmit antennas with appropriate weighting. The benefits of the weighting are to increase the signal gain and/or to reduce interference from other users of the communication system. Precoding requires knowledge of the Channel State Information (CSI) at the transmitter.
In wireless communication systems equipped with multiple antennas at both transmit and receive sides, a common problem is how to transmit signals in an optimal way. For example, if the wireless channel is known to the transmitter then one can employ optimal precoding over the channel modes combined with power allocation according to the well-known waterfilling principle.
Waterfilling in wireless MIMO systems is based on the assumption of a total transmit power constraint. The waterfilling solution often allocates all of the total transmit power over only one or a few transmit antennas or PA's (power amplifiers) driving the transmit antennas. In practice, the total transmit power constraint typically leads to infeasible solutions since its power distribution may exceed individual output power limitations of transmit signals or PA's. Further, the waterfilling solution is an iterative process that consumes valuable data processing power.